Repair of a Trio of Audiolab 8000A Amplifiers
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- Опубликовано: 8 сен 2024
- Three amplifiers, all with different faults, We deal with them one by one. Replace dry capacitors and go through the bias setting routine. The phono connectors all needed replacing and we even get creative finding a solution for some missing buttons.
Serial numbers of these units:
8953254
207B82184
207E115261
I ajusted recently the bias current on my Audiolab 8000A (version of 1988, issue 3).
May be my particular personal experience doing this can be useful to someone. The amplifier runned very hot. So I'm sharing this experience. Myalbatech showed this in his beautiful video "Repair of a trio of audiolabs" and the following is my particular personal experience.
This version has not presets and bias current is set by the pair of SOT resistors R809//R811 on L channel, and R810//R812 on R channel. In each channel, the pairs of resistors are connected in paralel (//). In my amplifier, R809 was 6k8 and R811 was 43k. Also, R810 was 6k8 and R812 was 43k.
Please note the pairs above are also in parallel with another resistor (placed on component side) of 1k5 , labelled as R807 on L channel and R 808 on channel R. Then (considering the 1k5 resistor) changes on the 43k resistor have very very small effect in the bias current. After some tests, performed as stated in the service manual, I set at 7k5 for R809 (L) and 7k15 for R810 (R), both preferred values, the first on E24 series and the second on E48 series). The 43k resistors were not changed.
SOT resistors were selected with the amplifier cover and baseplate mounted, but without screws fitted (as specified on the service manual). There is (by construction) a "heat chamber" inside the amplifier and this temperature rise inside controls the bias current in such a way it reduces slowly the bias, taking about 30 minutes to stabilize. As a curious test, taking the cover off the "heat chamber" lowers its temperature and the bias increases (ambient temperature was 26 ⁰C). So, a bias setting with cover off is not valid. And gives place to a too low bias when the amplifier is working with its cover in place.
Thanks for sharing a great video. I had one of these for my 40th ( 25 years ago! ) and it's still going strong. I've replaced some of the input sockets. I'll check my bias now, as it does get warm...
That was very educational, I thank you for such a detailed description of what you did to fix these three. You now answer question I had with my original 8000A as that ran hot. They are very old now guess it's to be expected component failures. Great video
Fascinating to watch this. I've had one of these since the late eighties and it's still going strong. It could probably do with a service (the volume knob is a bit wobbly since I shipped it to Shanghai 10 years ago and it sends some nasty pops through the speakers when switching off if I forget to turn the volume down ) but I'll probably wait until I move back to the UK next year as long as it lasts until then. 🙂
Hi Fras. If it's in use at the moment, it'd be worth just checking how much heat is coming out of the top, and checking whether there are any scorch marks on the circuit board. Once components start to go out of spec on these it's not unheard of for them to get so hot that they burn the board quite badly. I've not seen one myself, but apparently they can go far enough that they can be really hard to fix.
The pre drive transistors in these need additional cooing. I made heat sinks for them from springy screw clips, the type you get with car audio speakers so you can clip the clips to the door panels in order to have a self tapper screw into them. However you can buy small heat sinks for various different package / size transistors.
thoroughly enjoyed watching the video. ❤👌👍
I learned the hard way not to always touch heatsinks in amplifiers when feeling for warmth. I was working on a cloud cv 1000 where each side of each board has a live heatsink, if you touch both heatsinks at once you will get a huge shock and blow part of the amp. I believe i shorted the small transistor that helps to regulate the bias, it was a ZTX304 which can be used in pairs as a pre drive transistor but, in this case it is bolted to the side of the heatsink and regulates the bias as temperature changes.
You are not alone. I learned that kind of "lesson" in 1970, when I was making a tube amplifier based on EL 84 output tubes. I touched accidentally the socket joint to the anode: 140 V dc. Horrible and very painfull.
Nice to see another video and with Audiolabs too 🙂
Thank you, this was very helpful as I'm working on an 8000A running too hot. Unfortunately it's a bit of a nightmare since my board doesn't have component marking on the solder side, and on the top side the transistors are marked '1', '5' etc (like the first one in the video) i.e. no relation to the schematic. Resistors may be marked but underneath so you need to remove them to see it!! I guess I have an old board but the s/n is 89... I wonder what I'm missing.
Amazing work on the replacement buttons!
Hi, myalbatech,
In the repair you make in "repair of a trio of Audiolabs 8000a" what was the value of the variable multiturn resistor (preset) you used in the adjustment? 4.7kOhm? 10kOhm? 22kOhm?
I can't read it on your video.
After finding the correct value, the chosen resistor is soldered on the component side or on de copper side of the PCB?
Many thanks.
It was probably 10k though the value isn't really important as long as we can add a few k to whats already there. Resistors are then fitted on solder side of the board.
Good morning,
When adjusting bias in the Audiolab 8000a amplifier, we need (or do not) have 8 ohms loads connected at the amplifier outputs?
Could we adjust bias with output binding posts not connected to speakers or suitable resistive loads?
It should not matter either way.
@@myalbatech , Thank you very much.
Nobody seems interested in the pcb drawings of the Audiolab 8000A pcb.
I mean the drawings of the circuit board itself, from component side and from copper side, with components positions and labellings. They are not available in the 207 technical manual pages we usually obtain. If you want, please send me a private e-mail and I will send them for you.
Best Regards
Guilherme
@@guilhermedealmeida2299 I'm sure folks will pop up when they have a need.
Hi,
There is one important question not yet explained.
OK, as we can see from the service manual ( bias section) the resistor pairs R809/R811 and R810/R 812 control bias current in the final big output transistors. And thus bias current can be monitored mesuring the tension across the double emitter resistors (0.22 ohm + 0 22 ohm).
But in what direction the changes in R809/R811 and R810/R 812 change or affect bias current? Reducing R809 ou R810 will increase or reduce bias current?
Many thanks.
Guilherme de Almeida
Its somewhat non intuitive but increasing one or both resistors reduces the bias. You can put a variable resistor in series with the largest resistor of the pair and adjust to find the ideal value. Often simply moving the higher value up one preferred value is a close enough answer.
Hi,
Thank you very very much.
It seems a very short space to insert a preset variable resistor jn the component side.
I saw in the circuit diagram of the 8000S model (very similar circuit topology) Audiolab used, in paralel with the 1k5 resistor already in circuit (R807 in L channel), a series formed by a 3k9 fixed resistor and 4k7 preset ( in the place of R 809).
And R 811 do not exists. These are resistor numbers for L channel (respectively R808, R810, R812 for R channel).
Regards.
Guilherme de Almeida
To clarify - when I say put in a variable resistor I mean temporarily. Adjust to perfection then take it out, measure it and then pick the nearest value of fixed resistor to install.
OK, but the temporary variable resistor, e.g. 4k7 or 10k needs to have space, even temporary.
Aparently, following the Service Manual, it seems they want the service be done running two long wires from the positions of the terminals of the SOT resistors (resistors already removed) passing below the amplifier cover and testing different resistors at the free ends of these two wires, looking for the correct bias. At same time, two other wires connect the terminals of the double emitter resistors (2×0.22 ohms) to an external multimeter also outside the amplifier box.
It also seems the amp designer wants the changes for SOT resistor optimization be done with the amp ON, following at same time the voltage across the emitter resistor in order to obtain 22 mV after waiting for about 5 minutes with box closed...
Too much complication...
Beware of charred boards. if there is carbonisation then the board will start to conduct electricity.
Is there not a danger of the carbon in the burnt areas conducting?
There will be some resistance over any carbonised material and we do need to check for leakage. It wasn't an issue for the circuitry concerned here.
Hi,
This is still about the bias adjustment procedure on Audiolab 8000 A. One curious question... Did you also see this?.
As long as I can see, in page 4 of the Audiolab 8000A service Manual ("Service Manual 207, serial numbers 87... & 89..."), line 10, we can read: " See drawing CST207-036".
However, my Service Manual finishes at page 17 with the drawing CST207-028. There is no drawing CST207-036. Is my manual incomplete or Audiolab forgot to include such drawing?
Regards.
Guilherme
I have the 8000A that I bought used plus a 8000p power amp. They both work well but is there anything to look out for?
In every one I've seen the Bias has drifted high
@@myalbatech thanks, I'll have to test it.
Cheers bud have you a link for the rcas you used for these ?
You can find them on Ali express
I have an early version with 4 to220 outputs per channel (not the darlington sankens)... Does anyone have a schematic for the early mk1's?
i have a manual for serial nummers 87.... & 89.... if that helps. it says model 207.
Hi,
I suceeded obtaining the 2 pages in fault on the service data manual for the Audiolab 8000a (drawings CST207-036). The famous pages with the pcb drawings as seen from the component side and also for the copper side. Someone interested please send me a private e-mail.
Measuring the voltage across each one of the reservoir power supply ELNA capacitors (10000uF/50V), with a high impedance digital multimeter as a voltmeter, about 45V is a regular value? In each one (diagram says +44V and -44V).
The big problem is my 8000A is labeled at rear as 220V but in my country ( Portugal, Europe) I measure normally a mains AC voltage in the range 231V-233V. By the local standard it is nominally 230 V, with extreme possible limits at -11% and +6%. I'm worried if this may overload the amplifier, stress components by heat... And also a voltage dangerously near the 50V capacitor range.
No one interested?
No, the service Manual doesn'thave these 2 specific pages. When reading the part concerning the bias adjustmente, they refer these pages but that specific pages are not present in the 207 Manual.
Witam mam pytanie co trzeba rozpiąć w środku wzmacniacza by rozdzielić przedwzmacniacz z koncowka mocy ?
The schematics for this are online.
za wiele wlasnie nie ma dobra dzieki
Witam mam jeszcze pytanie które rezystory trzeba wymienić by obniżyć prąd spoczynkowy tak zwany bias?
How can I get in touch for cyrus servicing please?
Email me, address at end of video's and under the 'about' tab in RUclips
Nice